The present invention relates to data filtering, and more particularly, to a loop filter for reducing blocking effect and ringing noise, and to a loop filtering method.
Generally, picture coding standards such as MPEG of the International Standardization Organization (ISO) and H.263 of the International Telecommunication Union (ITU) use block-based motion estimation and discrete cosine transform (DCT) of blocks. Block-based coding causes a blocking effect and ringing noise, as is well known, when an image is highly compressed. Typical blocking effects include grid noise in the homogeneous area having relatively similar pixel values between adjacent pixels and staircase noise, in which an image edge is shown in the shape of a staircase along the edge of the image. Also ringing noise, a typical Gibb's phenomenon, occurs when a DCT coefficient is truncated by quantization in order to highly compress the image.
In the case of grid noise, traces caused by the process performed based on the block appear at the border between blocks when the compressed data is restored for display on a screen, so that the border between blocks can be noticed by a user. In the case of staircase noise, the edge of the image has the shape of a staircase, such that the bumpy edge of the image is noticed by a user. Also, ringing noise causes a problem in that a plurality of pictures are shown as being overlapped in intervals.
Methods have been proposed to solve the blocking effect and the ringing noise occurring during a block-based coding process. First, according to H.261, the blocking effect is decreased using a simple 3.times.3 low pass filter as a loop filter. Also, a simple edge loop filter has been suggested for reducing the blocking effect and mosquito noise. The edge loop filter linearizes values of two adjacent pixels positioned at the border of blocks to replace two pixel values by the linearized values. However, the edge loop filter cannot reduce the ringing noise even though the blocking noise is reduced. Also, a non-linear filter adopting a binary index has been suggested for reducing the ringing noise. However, the non-linear filter also cannot solve the problem related to the blocking effect.